qmlviewer

Where the -I is the path to the plasma plugin for qml. Try to look for the path of
/usr/lib/kde4/imports/org/kde/plasma/graphicswidgets/libgraphicswidgetsbindingsplugin.so
and use everything up to org of that path.

Hovewer it's strongly discouraged to use qmlviewer to develop plasmoids, because some features won't be available there: